I grab teleport as Irelia since A) free farm in other lanes later yaaaaay, I need that shit, and B) if their jungler actually manages to kill me, with both of them in lane I pretty much lose my tower no matter what time of the game it is. Teleport means I can get there and prevent it.

1. Flat armor penetration is not percentage based. It is a flat number.
2. The arpen MASTERY, however, is percentage based, as is arpen from every item.
3. The calculation for armor damage reduction is 100/(100+armor). Flat armor penetration is calculated FIRST, and % based calculation is done SECOND. Meaning flat arpen is essentially a wasted rune, because flat arpen runes will devalue % based runes.

Flat AD is good for several reasons. Mainly, however, it makes your early game ridiculously good.

Assuming 10% armor penetration, any AD damage will do true damage to melee minions until their armor reaches ~60, so about 10 minutes into the game. Likewise, AD damage will deal true damage to caster minions until about 16 minutes into the game.

Clearly, flat AD runes are better for last hitting. From purely a CS standpoint, arpen runes would be useless because you can't do MORE than true damage, and you'll do true damage for a long time with just the masteries. And again, flat arpen runes do not calculate very well.

Against a Cait with the same armor runes and masteries, you'll reduce her armor to about 25% damage reduction. However, Urgot will now be doing 51.75 damage with each auto-attack and 102.2 damage Acid Hunters at level 1.

And guess what? % Arpen scales with their armor. Flat ArPen doesn't scale at all. Meaning in Scenario 2, Urgot will continue to do high damage to Cait even when she buys armor. In Scenario 1, as Cait buys armor, Urgot just gets sad because his flat arpen doesn't do shit.

tl;dr Armor penetration runes are terrible now, run flat AD instead. Also, I have too much time on my hands.

tl;dr Armor penetration runes are terrible now, run flat AD instead. Also, I have too much time on my hands.

I'm sure the maths is wrong I'm just too lazy to check it.
a cait with armor seals + quints has 33% damage reduction and 48 armor at level 1.
41 arpen would reduce that to 7, then the 10% to 6 rounded down. This is 6% damage reduction, not 21%. Q does 50.149 assuming no ad increase.

on the other hand, just the 10% would reduce that 48 armor to a cool 44.2. This is 31% damage reduction. you then have an extra 16 AD. Q does 44.9905 at level 1

of course, it will fall off late game, but thats a whole different story
thing is, urgot with 41 arpen does TRUE DAMAGE to anything with below 41 armor. Which, if I'm right, happens to be everyone in terms of base armor. On the other hand, that 10% will only chop off 1 or 2 and a decimal.

unless I completely fucked up my maths (incoming boxes because i suck at theorycrafting), flat arpen gives you a better edge early game, not flat ad.

I'm sure the maths is wrong I'm just too lazy to check it.
a cait with armor seals + quints has 33% damage reduction and 48 armor at level 1.
41 arpen would reduce that to 7, then the 10% to 6 rounded down. This is 6% damage reduction, not 21%. Q does 50.149 assuming no ad increase.

on the other hand, just the 10% would reduce that 48 armor to a cool 44.2. This is 31% damage reduction. you then have an extra 16 AD. Q does 44.9905 at level 1

of course, it will fall off late game, but thats a whole different story
thing is, urgot with 41 arpen does TRUE DAMAGE to anything with below 41 armor. Which, if I'm right, happens to be everyone in terms of base armor. On the other hand, that 10% will only chop off 1 or 2 and a decimal.

unless I completely fucked up my maths (incoming boxes because i suck at theorycrafting), flat arpen gives you a better edge early game, not flat ad.

Flat arpen gives you the damage advantage for roughly two levels, by which point you're doing diminishing damage to champions (and the same damage to minions as before, obviously).

In theory, flat arpen could be used on a champion playing hyper aggressive, but you'd need a kill before level 3 or 4, because you'd need increasing AD or arpen to back that up. Otherwise it's not worth it.

I suppose you could say it's situational, but AD is arguably better the majority of the time.

urgot needs kills or lane advantage early game though in order to not fall off as much when lategame rears its head.

also one thing I'd like to say about urgot is that you're probably better off with ignite over ghost. When you ult, if you do it properly, the person you ulted has to go past you, or into ignite range. That's when you slap it on them and wail into them.

Give me a case where not having flat arpen would result in a higher reduction than having it.

The point isn't that flat arpen doesn't reduce armor, the point is that flat arpen is negligible compared to % based, and if you replace flat arpen with AD you might not have the same armor penetration but you'll more than make up the damage with the extra AD, even with lower arpen.